No. 4 "Compensation" Tests

I was thinking about this issue, and considering how you can differentiate between the error caused by the velocity and the barrel deflection?

So I have an idea, but it's open for debate, to see if it would be worthwhile.

So assuming the rifle bedding and the receiver rigidity/flexing is causing one part of the vertical spread, and the velocity differential is also causing part of the spread. Can we lay the rifle 90° to vertical, and test it? 

The idea being that the barrel deflection should present itself as a sideways error, and the velocity differential would remain in the vertical plane. 

So, it's just an idea, would it work?

If a difference in receiver rigidity and flexing is creating the barrel to deflect ,  why is it only in the vertical plane?

It would seem logical ( to me) that a flex in the receiver caused by the difference in strength of the left and right sides, would cause a deflection in the horizontal plane. Does the upward pressure of the barrel and at the knox form cause it to always be vertical?

To solve the problem, we need to understand exactly what is causing the deflection?

 It an interesting subject, which it seems, no one has really been able to diagnose completely. We know it exists, we know it is not in every rifle.

Geoff, if your No4mk1 T has the same issue with the different barrels, that makes it very interesting!

Velocity variations were particularly high with 7.62 NATO ball ammunition in the 1960's when the No. 4 7.62 conversions were first being used at Bisley and Connaught.  What they saw was good scores at long range (900 and 1000 yds) but very poor grouping at short range (200 and 300 yds).  Positive compensation would have this very effect with large velocity spreads.   

I also suspect that the higher muzzle velocity of the 147gr 7.62 bullet (compared to the 174 gr .303 bullet) resulted in the 7.62 bullet leaving the muzzle at the moment when the barrel was quickly moving upwards.  The .303 bullet travelling slower may have been been close to the point of maximum upward barrel deflection when the barrel movement had considerably slowed down.  What causes positive compensation is a rapid upward movement of the barrel when a bullet at nominal velocity exists the muzzle, and variations in velocity result in different angles of departure of the slow and fast bullets.

All this conjecture and theory may be interesting (well, maybe only for me!), but I believe this can be proven, perhaps only indirectly.  

In the Table I generated earlier in this thread, I show the delta bullet Elevation at 600 and 1000 yards for a 100 fps velocity extreme spread.  That was 2.2 MOA and 5.2 MOA respectively.  Now, for a rifle that fully compensates for velocity variations (like my No. 4 7.62 does at 800 yards), the positive compensation effect is equivalent to the bullet elevation variation (and in the opposite direction).  Ergo, something approaching 5 MOA!  A 5 MOA elevation change for 100 fps velocity change would be dramatic at 200 yards where bullet drop trajectory change is just 1/2 MOA for 100 fps velocity change.  5.0 - 0.5 = 4.5 MOA.  And that's about exactly what has been reported during the 1960's for group elevation values at 200 yards with the No. 4 7.62.  The 200 yd Bisley target at the time had a 2.5 MOA bull, and if your ammo was giving velocity spreads of 100 fps, you would see elevation spreads of 4.5 MOA, not very satisfying.  I've experienced this myself with some poor quality 1960's DAC 7.62 Nato ball ammo which I discovered had velocity variations of 100 fps and more.  I pulled the bullets on some of this ammo and found powder charges varied by as much as 3 grains!

The testing I intend to do this weekend should be interesting.  I'll load up .303 ammo with 40.0 and 42.0 gr Varget, 174 SMK, and shoot the T with and without the scope at 200 yards to see how much elevation change occurs as a function of velocity change and in what direction it is.  My prediction?  Negative compensation with the scope attached (fast bullets hit substantially > 0.5 MOA higher than slow bullets) and perhaps neutral or slight positive compensation (fast bullets hit about the same elevation as slow bullets) without the scope. 

     

I’m probably getting an average ES of 40-50 fps.  But also a fair number in the 30-40 fps range too, and these group well at 600 yds.   But, I’ve had perhaps 3 or 4 ten shot groups at 90-100 fps! This with weighing out charges! 

Virtually all of this data was with WLR primers.  I did a test with CCI BR-2’s and it gave me higher ES and SD values.  I will try regular CCI 200’s next time. 

Addition of the No. 32 Scope alters Muzzle Jump substantially compared to iron sights and results in Negative Compensation.  With a 174 gr bullet at a muzzle velocity of 2400 to 2500 fps, the No. 4 T Negative Compensation increases elevation spreads by nearly 0.2 MOA for every 10 fps velocity change.  This combines with bullet drop effects to result in a total elevation spread of 0.4 MOA for every 10 fps velocity change at 600 yds. 

The first test was to determine the degree of Compensation with the scope removed. To do this, I fired two different loads at 200 yards. Both loads with PPU cases, 174 gr SMK and WLR primers. 

Load 1:  40.3 gr Varget

Load 2:  41.5 gr Varget

After initial sighting in, I fired a 10 shot group of Load 1.  I had one shot with a low velocity of 2343 fps, so discarded that from the muzzle velocity statistics and calculation of group MPI:

Load 1 Results (9 shots): 

   MV = 2405 fps

   ES = 26.9 fps

   Elevation Mean POI = -0.20 inches below target center.

Load 2 Results (five shots):

   MV = 2476 fps

   ES = 40.9 fps

   Elevation Mean POI  = + 1.56 inches above target center

The resulting elevation difference is +1.76 inches (+ 0.88 MOA) for a 71 fps muzzle velocity increase.  From my Strelok trajectory calculations, the elevation POI rise for +75 fps muzzle velocity increase at 200 yards is +0.40 MOA. Therefore, this rifle without the scope has a slight Negative Compensation of 0.88 - 0.40 = + 0.48 MOA for a 71 fps muzzle velocity increase.  This works out to + 0.07 MOA rise in elevation per 10 fps velocity increase, excluding bullet drop effects.   So even a 100 fps velocity ES at this range would not result in elevation spreads much more than 1 MOA, making it a good short range shooter. 

However, what I did find is that at the higher muzzle velocities of Load 2, the bullet POI of the five shots taken were very flat, less than 0.2 MOA vertical spread for a ES of 41 fps.  For Load 2, the fast bullet hit low and slowest bullet hit highest on the target. This indicates the 41.5 gr charge is at an accuracy node and the barrel is likely near the top of its vertical rise, whereas bullets leaving the muzzle with the lower 40.3 gr charge occur later when the muzzle is moving downward.  At muzzle velocities between 2450 to 2490 fps, the rifle compensates just beyond 200 yds. 

The above test was repeated with the No. 32 scope installed. The same loads were fired at 200 yards giving the following results:

Load 1 Results (five shots): 

  MV = 2415 fps

  ES = 29.5 fps

  Elevation Mean POI = + 0.81 inches above target center 

Load 2 Results (10 shots)

  MV = 2482 fps

  ES = 59.5 fps 

  Elevation Mean POI = +3.94 inches above target center

The resulting elevation difference with the scope fitted is 3.94 - 0.81 = +3.13 inches at 200 yards (+ 1.57 MOA) for a muzzle velocity change of 67 fps. Now, subtracting the contribution of the elevation rise due to change in trajectory bullet drop of approx 0.35 MOA, we get a net +1.22 MOA elevation rise for 67 fps velocity change for a negative Compensation effect of +0.18 MOA elevation rise per 10 fps velocity increase.  This is more than 2X the elevation rise as compared to the scope removed. 

I think what is happening is the added stiffness and mass to the action body from the scope and mount shortens the time it takes for the muzzle to rise and reach its first peak.  Bullets are leaving the muzzle after the first deflection peak as the muzzle is moving downward., giving Negative Compensation.  The options are to increase muzzle velocity and see if I can get the bullets to exit the muzzle on or before reaching the peak of muzzle rise. The other option is to experiment with barrel bearings/packing to alter the timing of muzzle rise/fall.  

I may try a 150 grain bullet to get the muzzle velocity over 2700 fps, this may prove the theory.  

More to do. 

200 yard is a short range for this rifle and these results have convinced me the rifle with scope indeed has negative compensation. But I already knew that by comparing 600 yd elevation spreads to POI elevation spreads at the target. This test allowed me to quantify it by removing most of the bullet drop by shooting at short range rather than 600 yds.  My elevation error at 600 yards is entirely due to velocity variations, as bullets with the same velocity stack on top of each other at 200, 300 and 600 yds. Looking back at the plot sheets, very good groups only occur when velocity extreme spreads are less than 30 fps. 

This figure below shows what is occurring.  Without the scope attached, bullets faster than about 2400 fps exit the muzzle on the left side of the peak, along the blue line, showing positive compensation.  Bullets slower than 2400 fps exit on the right side of the peak, showing negative compensation. 

With the scope attached, the increased stiffness is making the muzzle rise sooner, and this entire muzzle projection curve is shifted to the left such that bullets slower than 2500 fps exit on the downward side of the peak along the red line.  I don’t know about bullets faster than 2500 as I’ve not pushed my loads that fast.  Question is, what muzzle velocity do I need to get them to exit close to the top of this curve and preferably on the left side of the peak? 

It’s possible that shooting with a sling rather than the forend rested on a sandbag and pulling the rifle into my shoulder might change the rifle dynamics enough to make a difference.  I will also try 150 gr bullets at higher velocity to see if I can get them to exit the muzzle sooner along the blue line. 

The compensation with the scope, although negative, is not nearly as much as what the No. 4 7.62 rifles experienced back in the 1960’s which were particularly bad at short ranges.  A rifle that fully compensates for bullet drop at 1000 yards will have vertical spreads as much as 5 MOA at short range for muzzle velocity spreads of 100 fps. Very poor ammunition with an ES of 200 fps will be double that! 

I dug up some early notes I made on my Fulton No. 4 and it also possessed negative compensation with a scope fitted.  At the time, I did not realize this, and I did not have a chrono. The elevation rise at 100 yards was 0.83 inches per grain of powder (approx 50 fps increase in MV per grain) and very linear over a 3 grain charge range.  If the rifle had positive compensation, the POI would have decreased for an increasing powder charge.